Raiders wide receiver Javon Walkerhad knee surgery sometime last month. A few days ago, he finally let team officials in on the secret.

That explains why Walker is not practicing at the team's mandatory minicamp, which started Friday with two-a-days. He won't be ready to go until training camp, Raiders coach Tom Cablesaid.

"We just found out about it," Cable said. "Yeah, it is a little bit uncommon but we're aware of it now and it was probably the right thing to do and get it cleaned up."

It's the same right knee that he had surgically repaired in 2007 with the Broncos. He had season-ending surgery on his left ankle midway through last season, but that injury is healed, according to Cable.

The bigger concern is the lack of communication from a player who has stayed away from Oakland throughout the voluntary workout program this offseason.

His first year was troubled enough. He was beaten and robbed in Las Vegas during the offseason, offered to retire in training camp, and underperformed on the field (15 catches for 196 yards) after signing a six-year deal worth up to $55 million.

Walker agreed to a $27 million pay cut when he agreed to a restructured contract in January, the main reason the Raiders did not release him. But, he is still guaranteed $4.6 million in base salary the next two seasons.

With first-round draft pick Darrius Heyward-Bey running with the first team, Walker may have a hard time hanging on if he doesn't have a strong training camp at this point.

NEW RACE: The Raiders signed four-time Pro Bowl fullback Lorenzo Nealjust in time for Friday's minicamp.

BRIEFLY: Tight end Zach Milleris recovering from surgery, though the team knew about his sports hernia procedure from last month. New center Samson Sateleis also out because of shoulder surgery. ... Wide receiver Arman Shieldshas yet to fully recover from last summer's knee injury and is out. Lastly, defensive end Derrick Burgessdid not practice because of the stomach flu. ... The Raiders signed former Steelers running back Gary Russell.